fix() method for FreeAmplitude's

I think we should implement a `FreeAmplitude::fix()` method so that
```cpp
free_amplitude(*D, to(rho))->variableStatus() = VariableStatus::fixed;
*free_amplitude(*D, to(rho)) = std::complex<double>(1., 0.);
```
can be replaced by
```cpp
free_amplitude(*D, to(rho))->fix(std::complex<double>(1., 0.));
```